1. Field of the Invention
The present invention relates generally to an integrated circuit resistor array used for providing a number of resistors within an analog integrated circuit which may affect the voltage dependency of a predetermined parameter such as a gain of the analog integrated circuit, and more particularly to a resistor array which is suitable for use as a number of such resistors within an analog integrated circuit requiring high performance, such as a summing amplifier in a digital-to-analog converter.
2. Description of Related Art
A conventional oversampling multi-bit digital-to-analog converter (DAC), for example, model PCM1710 DAC manufactured by the assignee of the present application, generally includes a noise shaping circuit for oversampling a digital input, a plurality of CMOS inverters the outputs of which are switched to high or low in response to a multi-bit digital modulated output from the noise shaping circuit, and a summing amplifier for combining outputs from the inverters to generate an analog output. The summing amplifier comprises a plurality of converting resistors (i.e., unit weighting resistors having a uniform weight) each having one of the terminals thereof connected to the output of a corresponding inverter, a CR filter connected to the other terminals of the converting resistors opposite to those connected to the inverter outputs, and a differential instrumentation amplifier connected to the output of the CR filter.
The integrated circuit of the model PCM1710 DAC includes within the summing amplifier a first group of resistors including the converting resistors as well as filter resistors in the CR filter and a second group of resistors including input resistors and feedback resistors in the instrumentation amplifier. The first and second groups of resistors are provided by poly-silicon resistors which are located in suitable positions near their associated circuits and thus in first and second regions spaced apart from each other. It should be noted that "poly-silicon resistors" referred to in this disclosure include not only resistors made of poly-silicon material only but also resistors having a polyside structure. The term "polyside" is commonly used to refer to a stacked film layer consisting of a polycrystalline silicon layer and a metal layer.
Each poly-silicon resistor in each of the first and second regions is constituted by a stripe arranged in a comb shape. The comb shape stripe consists of a plurality of elongated tooth-like poly-silicon stripe portions and a plurality of short connecting poly-silicon stripe portions each of which connects one of the ends of a tooth-like stripe portion to one of the ends of an adjacent tooth-like stripe portion. The connecting stripe portions extend in the direction orthogonal to the tooth-like stripe portion. The tooth-like stripe portions and connecting stripe portions in each of the first and second regions have the same thickness, width and length. Also, the tooth-like stripe portions in the first region are positioned orthogonal to those in the second region. In the structure mentioned above, since a required resistance of a resistor is determined by a total length of the comb shaped stripe formed by a serial connection of adjacent tooth-like stripe portions made by connecting stripe portions, resistors having different resistances may be realized in the same region by serially connecting different numbers of tooth-like stripe portions through connecting stripe portions.
The above-mentioned poly-silicon resistors are advantageous over metal thin-film resistors made of silicon chrome, nichrome or the like from a viewpoint of process contamination and process adaptability in the sub-micron integrated circuit technology. However, the poly-silicon resistor has a problem that its voltage dependency (or voltage coefficient) is higher as compared with the metal thin-film resistor. Also, the poly-silicon resistor has a characteristic such that its voltage coefficient is smaller as its width is narrower with the same thickness, i.e., as its cross-sectional area is smaller.
Further, the integrated circuit process techniques, particularly, the etching of poly-silicon or the like has a problem that the etching rate depends on etched location, etching direction and etched area on a chip. Thus, even if a poly-silicon material is to be etched to form poly-silicon resistors having the same width in different locations, in different directions, or over different etched areas, poly-silicon resistors having different widths may result, whereby the poly-silicon resistors would present different voltage dependencies. As a result, parameters of the summing amplifier, for example, a gain, would have a voltage dependency which is dependent upon the voltage dependencies of the poly-silicon resistors. In view of the characteristics of the whole DAC, this would adversely affect analog characteristics of the DAC such as linearity, dynamic range, and so on.
The above-mentioned problem may apply not only to an analog integrated circuit such as a summing amplifier in a DAC but also to analog integrated circuits within other devices.